Modern commercial and military systems such as radar systems, and satellite communication systems, often perform multiple functions that can require a plurality of different radar beams at different wavelengths. Examples of these functions include surveillance of targets and objects at various ranges/distances, air traffic control, navigation, weapons control, weather surveillance, satellite uplink and downlink signaling, telecommunications, and Internet communications. In many of the environments in which such systems are deployed, it can be difficult to provide multiple antennas to support the multiple different beams because of space and/or cost limitations. Consequently, it is advantageous to employ a phased array antenna in such environments.
As is well-known, a single phased-array antenna can simultaneously radiate and receive multiple radar beams, because of its control of the phase of multiple radiating elements. One complicating factor in design of phased arrays, however, is that many radar functions require simultaneous availability of beams spanning two or more radar bands. For example, long-range surveillance conventionally requires longer wavelengths (λ), e.g., S band, whereas precision-tracking and target-recognition radars generally operate most efficiently at shorter wavelengths, e.g., C band. Weapons control and Doppler navigation are typically performed at still shorter wavelengths, e.g., X band and Ku band. However, for systems that require wide scan angle such as ±60° from boresight, combining radiating elements of two bands into a single aperture is a real challenge because of the constraints on element spacing and size. Furthermore, providing isolation between the two bands can be difficult and, as further explained below, it is possible to have interference and cross-coupling between the beams of the two different bands.
Phased array designs are typically limited in element spacing and size to avoid grating lobes. For example, some conventional phased array elements are approximately λ/2 apart and can occupy the entire space allocated to an element in a wide angle scanned array. If such conventional elements are spaced at greater than λ/2 wavelengths, the power of the radar signals can divide and, at wide scan angles, grating lobes can occur: as the beam is scanned further from broadside, a point is reached at which a second symmetrical main lobe (grating lobe) is developed. This unwanted condition can reduce antenna gain by several decibels (dBs) due to the second lobe. For dual-band military applications in particular, grating lobes can be a problem because the broad frequency bandwidth requirements mean that at the high end of the frequency band, the elements may be spaced greater than λ/2. The presence of grating lobes can cause a radar system to produce ambiguous responses to a radar target. Such a radar system also can be more prone to interference.
Still another bandwidth issue for phased array designs is the problem of beam distortion with scan angle. Beam distortion with scan angle results in spread of the beam shape and a consequent reduction in gain known as “scan loss”. For an ideal array element, scan loss is equal to the aperture size reduction (projected) in the scan direction, which varies based at least in part on the scan angle.
An additional complicating factor in the design of antenna elements, including elements for phased arrays, involves transitions between different types of transmission lines in the system. In many high frequency systems, it is necessary to implement part of the system in coaxial transmission lines and another part of the system in waveguide transmission systems. To transfer signals from one of these mediums to the other, a coaxial transmission line to waveguide adaptor (also referred to as a coax to waveguide transition) is provided. Waveguide to coax transitions are known in the art, where the waveguide is a thin rectangular member having conductive surfaces, and the coax includes an inner pin conductor and an outer conductor. Generally, the output of the transition contains the configuration of a conventional waveguide type transmission line; the input of the transition contains the structure of the conventional coaxial type transmission line containing a central conductor surrounded by a dielectric.
FIG. 1 is an illustration of a prior art design using a conventional waveguide to coaxial transition 12. Referring briefly to FIG. 1, the transition 12 is coupled to a coaxial connector 14 having a central conductor 16 surrounded by a dielectric material (not shown in FIG. 1). The impedance matching section 10 is connected to a waveguide 18, which is illustrated in FIG. 1 as being substantially rectangular with a tapered section. The waveguide 18 includes a first section 20 filled with air and a second section 22 filed with dielectric material, where the second section in this example embodiment includes a tapered portion 22A extending into the air section. Dielectric material is used to reduce the size of the waveguide and the tapers on both waveguide and dielectric sections are designed to ensure good impedance matching.
In known transition implementations from waveguide to the coax, such as the transition 12 shown in FIG. 1, the outer conductor (not shown) of the coax 14 is electrically connected to one conductive surface of the waveguide 18, and the inner conductor 16 of the coax 14 extends into the waveguide and sometimes is loaded with a small dielectric or metallic disk at the end to increase its capacitance for better impedance matching. The electromagnetic waves from the antenna impinge on the inner conductor 16 and induce a current that is directed to a circuit operably connected to the coax 14.
Still referring to FIG. 1, receiving antennas collect electromagnetic energy from the free space 23 for reception purposes, and a receiver or other processing circuit coupled to the antenna detects and processes the collected energy. For certain frequency bands, waveguides 18 direct the radiation that the antenna collects to the receiver or other processing circuit. The radiation generally travels in free space 23 through the waveguide 18, and is collected by a coaxial connection 14 that is electrically connected to the receiver circuit. Often, the receiver circuit and the waveguide 18 are very different in size, so the waveguide 18 includes an adapter 12 and/or one or more transitions to reduce its size from the antenna to the coaxial connection 14. The various transitions through the waveguide 18, including the transition from the air waveguide 20 to the coaxial connection 14, preferably are such that the transitions are impedance matched to limit the losses of the collected radiation to a minimum.
In addition, as shown in FIG. 1, the dielectric material 22 filling the waveguide helps to provide a further transition and impedance matching. As is known in the art, by filling the waveguide 18 with dielectric material 22 having a relative permittivity greater than 1, the width of the waveguide 18 can be reduced significantly in its operating band. To ensure a smooth transition and good impedance matching between open-air waveguide and dielectric-loaded waveguide, taper sections for both waveguide and dielectric are commonly used.
In known implementations, the coax-to-waveguide adaptors are typically larger than the space available in the phased array environment. Again, this is mainly due to the element spacing constraint to avoid grating lobes. Another challenge is that elements having a narrow aperture generally have a higher impedance and it is harder to provide an impedance match to free space over a large scan angle.